A complex integrated circuit (IC) device often includes test circuitry. Such circuitry may be used to test the device at the end of the manufacturing line, or may be used after the device is shipped and installed by a system integrator to ensure continued proper operation of the device.
A scan chain is sometimes used in the test circuitry. The scan chain typically includes a number of scan cells arranged such that a scan cell at a later stage of the chain receives the output of a scan cell at an earlier stage of the chain. A multiplexer located between the scan cells receives the prior cell's output and a functional bit from device logic that may be tested using the scan chain. When a scan is enabled, the multiplexer selects the output of the previous scan cell for input to the later scan cell. When the scan is disabled, the multiplexer selects the functional bit. Thus, test data may be serially loaded into the scan chain, and input to the logic under test. Internal values produced by the logic under test may then be serially shifted out of the scan chain for evaluation.